Emission shader color saturation clipping
Easiest to explain it with an example. Let's take fully saturated blue. At power 1.0, that is RGB 0.0, 0,0, 1.0. Let's boost power up to 2.0. We now have 0.0, 0.0, 2.0. This can't be display on our monitor, so it clips to 0.0, 0.0, 1.0. Essentially negating the power increase. We have the same blue we started with.

Now let's try 85% saturation, but still at full brightness, emitter power=1.0. That's RGB 0.15, 0.15, 1.0. Let's put the emitter up to 2.0. We now have RGB 0.3, 0.3, 2.0. That clips to 0.3, 0.3, 1.0. See what happened? We raised all the channels, but only R and G had room to go up, so they increase relative to blue, leaving you with a desaturated blue. This doesn't happen with 100% saturation, because R and G are 0 in that case and don't get scaled up when you boost the power.

Btw, on the subject of the glare node, it doesn't clip linearly. I've never looked into what it does exactly, but it does some sort of scaling or clamping to avoid changing saturation or outputting superwhite values. (usually that's done by scaling down all the 3 channels by some identical value that will leave you with brightest channel=1.0)